Oxygen Charge, Oxygen Pressure

In contrast to the alkali charge and temperature, the charge of oxygen plays a less

important role, as long as sufficient oxygen is present in the oxygen delignification

system. It is agreed that an oxygen charge between 20 and 30 kg bdt–1 is sufficient

to avoid any oxygen-based limitation of the delignification process. In the

literature, different values for specific oxygen consumption per unit kappa number

reduction (DO2/Dkappa number) are reported. According to laboratory studies,

the oxygen consumption per unit kappa number reduction varies from 0.5 to

0.6 kg bdt–1 [54,55]. The evaluation of industrial oxygen delignification plants

revealed oxygen consumption values of 1.4 kg bdt–1 per unit kappa number

decrease for softwoods, and 1.6 kg bdt–1 for hardwoods [56]. In a dissolving pulp

mill using unbleached beech acid sulfite pulp, the oxygen consumption was calculated

from the quantity and composition of the exhaust gas from the blow tank of

the oxygen delignification stage [57]. A “helium tracer technique” was applied to

control the oxygen consumption [31].

From these measurements it can be concluded that the specific oxygen consumption

rate amounts to approximately 1.0 kg per unit of kappa number

decrease.

With increasing temperature, the utilization of oxygen increases without significantly

improving the delignification efficiency. Furthermore, it is reported that the

increased oxygen consumption parallels the increased loss of pulp yield.

On the basis of detailed material balances, the amount of oxygen consumed

during an industrial oxygen delignification process was estimated [58]. The study

of Salmela and Alen indicates that part of the oxygen bound to the reaction products

originates from alkali (about 13%), part from molecular oxygen (about 33%),

and the major part from the pulp (about 54%) itself. The specific consumption of

molecular oxygen needed for the oxidation reactions is, however, limited to 0.6–

1.0 kg bdt–1, which is in good agreement with the results obtained from laboratory

studies. An increase in the oxygen charge primarily induces increased oxidation

reactions with dissolved organic and inorganic compounds.

Unlike the oxygen charge, the oxygen pressure significantly influences the degradation

rate (see Section 7.3.4). Model compound studies using phenolic b-aryl

ether confirmed the pronounced effect of oxygen pressure on degradation rate

(Fig. 7.44).

Commercial oxygen delignification plants typically use pressures in the range

400 to 870 kPa with medium consistency systems applying higher pressures as

compared to high consistency plants [59]. However, there is a clear trend to

further increase oxygen pressure as high as technically feasible, especially in twostage

operations.

7.3 Oxygen Delignification 707

0 20 40 60 80

oxygen pressure: 0.4 MPa 0.6 MPa 1.1 MPa

Remaining â-arylether [%]

Reaction time [min]

Fig. 7.44 Influence of oxygen pressure on the degradation

rate of a phenolic b-aryl ether compound at pH 11 and 100 °C

(according to [39]).

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